1. Field of the Invention
The present invention relates to a powder sensor for detecting powder such as photocopier toner.
2. Description of the Related Art
Toner used in a photocopier for example is more consumed according as the number of photocopies increases and hence the remaining amount thereof needs to be detected at all times so that toner is newly supplied if the remaining amount reduces to a proper amount. A powder sensor is known that detects the presence/absence of the powder for this end.
The powder sensor of Japanese Laid-Open Patent Publication No. 3-37592 includes a powder sensor element (two-terminal piezoelectric element), a sweep oscillator circuit connected via a resistor to an input of the powder sensor element, a phase comparison unit that performs a phase comparison between a terminal voltage of the powder sensor element and a drive pulse signal from the sweep oscillator circuit, and a phase discrimination unit that discriminates the result of this comparison to detect the presence/absence of the powder. Specifically, a detected phase difference is latched into a register e.g. as level 0 when it is 80° to 90° and as level 1 when 0° to 10° based on a previously set threshold value of 45°, to output a detection signal as a digital signal depending on the presence/absence of the powder.
The conventional detection method brings about no problem in the common environment. However, in a particular environment, e.g., in case a large vibration or shock is applied due to assembly, adjustment, or other factors of a photocopier, the conventional detection method may cause a temporary shift in the phase of the terminal voltage of the powder sensor element, which may result in an misjudgment that powder is absent regardless of the presence of powder depending on the extent of the shift. With the progress of size reduction in OA equipment such as the photocopier, the powder sensor element becomes susceptible to the influence of a vibration originating from a motor in the paper feed, which may be a cause of the misjudgment.